The invention relates to solar simulators for simulating radiation received from the sun in order to test solar collectors and to test the effect of solar radiation on materials.
In recent years there has been a rapid growth in the use of solar energy, especially in the use of flat solar collectors for receiving radiation from the sun and converting it to heat water for use in hot water heaters and the like. Numerous flat collectors have been developed and introduced to the residential and commercial markets. In order to accurately determine efficiency and reliability of the various solar collectors that have been introduced, it has been necessary to develop meaningful testing methods and standards in order to enable engineers to accurately measure performance of solor collectors in response to incident solar radiation in order to allow them to improve their designs and also to enable buyers of solar collectors to compare the performance of the various available solar collectors.
In the past, outdoor solar collector testing has been performed. However, varying and inclement weather conditions have prevented an acceptable degree of repeatability of test results obtained using the outdoor collector techniques. The wide variations in seasonal weather conditions throughout the United States and the rest of the world make it difficult or impossible to achieve uniform outdoor standardized testing conditions.
Therefore, it is an object of this invention to provide a device and method for accurately simulating solar radiation to be received by a solar collector.
In order to accurately test solar collectors, it would be desirable to be able to accurately simulate a solar day for any particular geographical location in which a solar collector might be installed outdoors and also to accurately simulate solar performance for any particular season, so that "worst case" and "best case" and typical performance of the collector so installed can be accurately predicted.
Therefore, it is another object of the invention to provide a solar simulator and method which are capable of accurately simulating a predetermined solar day.
Solar simulators including stationary arrays of lamps have been utilized for testing materials used in spacecraft for many years. In some instances, such solar simulators have been used for testing the performance of solar collectors. However, during a typical solar day, the amount of radiation received by the collector is not constant, since the angle of incidence and the intensity of radiation received from the sun varies from morning to evening. (Most solar collectors of the type through which hot water is circulated in order to heat the water are mounted in fixed positions, so the incoming solar radiation is perpendicular to the face of the collector only at noon during certain months.) The stationary solar lamp arrays are incapable of simulating this type of performance, even if the orientation of the plane of the radiation-receiving surface of the solar collector under test is varied, due to the fact that such variation causes different points on the radiation-receiving surface to be at unequal distances from the plane of the lamp array. This in turn causes the intensity of the received radiation to vary along the surface of the solar collector, producing thermal gradients that cause the performance of the collector being tested to be significantly different than would be the case if the incoming radiation were produced by a distant "point source".
Therefore, it is yet another object of the invention to provide a solar simulator and method of operating that provides varying angles of incidence of received radiation and yet avoids occurrances of undesired thermal gradients along the surface of the collector being tested.
In the past, the above mentioned stationary solar simulators have been operated to compensate for the non-variability of the angle of incidence relative to the radiation-receiving collector under test by reducing the amount of power delivered to the solar lamps, thereby decreasing the intensity of radiation received at the surface of the collector uniformly to simulate the reduction in intensity of received radiation that occurs as the angle of incidence of the received radiation increases. However, this approach has the disadvantage that power reductions in some types of solar lamps causes an "unnatural" shift in the frequency spectrum emitted thereby, resulting in inaccuracies in measuring the performance of the collector under test.
Therefore, it is yet another object of the invention to provide a solar simulator and method of testing that avoids shifts in the spectrum of radiation received by a collector being tested as solar performance is simulated.
An important factor in accurately simulating actual outdoor collector performance on a perfect day is the amount of re-radiation of energy from the surface of the collector back into space. The amount of re-radiation depends largely on a parameter referred to as the "sky temperature", which is typically ten to thirty degrees Fahrenheit below the ambient temperature around the collector under test.
Therefore, it is a further object of the invention to provide a solar simulator and operating method that accurately causes re-radiation of energy from the surface of a solar collector being tested.